Helicopters count with a considerable number of vibrations generated by diverse sources, that go from simple vibrations originated by the effect of aerodynamic forces during flight, as are the aeroelastic and aeromechanical instabilities, to greater range ones generated by the motor and the rotor. When analyzing the behaviour of the helicopter, it can be observed that the intensity of the vibrations is greater during the takeoff and the landing.
The effects caused by the vibration are divided in two categories: effects produced by aerodynamic causes and effects produced by mechanical causes. All these effects bring with them impacts of different classes, such as comfort and efficiency of the crew, the fatigue of the dynamic components and the structure of the apparatus or the accuracy and level of efficiency of the electronic equipment.
The helicopter's main rotor produce vibrations in ranges from 3 to 12 Hz and the frequency depend on the number of blades. This range is much lesser to the one produced by the tail's rotor, which induces vibrations between 20 and 25 Hz. The produced vibrations travel through the aircraft's fuselage in the form of low frequency vibrations and adversely affect the helicopter's controls, the operations sub-system and the comfort of the crew, mostly in aircrafts with bi-bladed rotors, like the UH-1H helicopter.
The vibrations can be defined as the oscillatory movements of the particles of the solid bodies around their equilibrium point. Technologically is almost impossible to build a machine that moves energy from one place to another without the operation to be accompanied by vibrations of some kind. As larger are the mobile parts, more possibilities there are that vibrations are produced that are generally accompanied by noise.
The effect of vibration on the human body depends of the amplitude and duration. Due to the relation of possible adverse effects on the human body, low frequency vibrations (ranges of 1-100 Hz) are the most harmful for the crew. When studying the behaviour of vibrations is important to take into account four aspects: magnitude, frequency, direction and duration.
So, one of the greater problems that are presented with the flying of helicopters is that the main rotor as well as the tail's rotor, generate vibrations that are directly transmitted to the flight personnel, especially pilots and co-pilots, all time that the chairs of these crew are joined directly to the beams of the mentioned rotors, which are part of the aircraft's chassis. In this sense, every time that the pilot flights the helicopter, the movement of the blades is going to be transmitted directly to his chair and therefore to his body, which leads to serious health problems in the future.
Due to this type of situation, the aerospace medicine specialized medical centres perform a continuous follow-up to the health and physical integrity of the pilots, making emphasis in the helicopter pilots, who are subjected, in greater extent, to the exposure of harmful vibrations. The main consequences observed in this group of people are generally found reflected in lumbar problems in a 63% and from them a high percentage of cases aggravate with the continuous exposure to the vibrations produced by the aircraft, even causing irreversible damages. In addition to influencing the lumbar zone of the pilot, vibrations also affect visual acuity, motor function and orientation, creating risks during the aircraft's operation.
In the same way, in the rotor aviation world and in the field of aerospace medicine, its been given special interest to the study of the vibrations of the helicopter and the effects these cause on the body of the pilots. Aerospace medicine focus their efforts in raising awareness to the pilots of the importance of maintaining a good posture during flight and in maintaining daily updated data bases of the crew that suffer ailments caused by the vibrations. Whilst, in another level, of design and development of technology, the efforts have been focused to measures that seek to minimize to the maximum the vibrations produced by the yoke of the engine and aircraft designs have been reformed to provide more pleasant flights.
According to the Colegio Oficial de Pilotos de la Aviación Comercial (COPAC) (Official School of Commercial Aviation Pilots), an investigation of the Austrian Army centred mainly on the helicopter chairs, taking into account the parameters of the International Organization for Standardization, ISO 2631, guide for the evaluation of human exposure to whole-body vibration. It was verified, for example, that the model Bell 212, very usual in coastal zones, vibrations in the floor under the chair reach 12 Hz flying at 100 knots.
When performing the analysis of the aircraft in real flight and of the transducers on the chair's panel, it was observed that the chair increased the vibration amplitude peak, meaning, the chair has a clear negative effect on the pilot occupying it. To correct it, the Austrian Army reduced the maximum flight velocity of the Bell 212 to 90 knots. For each helicopter, the vibration frequencies of the pilot's chair are different and are related with the number of blades of the rotor, for example, the estimated value for a bi-blade helicopter such as the UH-1H, is of 12 Hz.
On a study published on the magazine Aviator, the Danish Acoustic Institute performed in September 1986 a study on the physiological effects of the professional exposure to whole-body vibrations, with 10 pilots of Bell 212 and AS332 Super Puma which are medium sized helicopters of features nearly comparable with the UH-1H; in flights which daily duration is between 3 and 6 hours, gave as result that the helicopter's vibration, measured with a load filter according to ISO 2631, is very near to a pure sinusoidal exposure with the spin frequency of the blades, in the case of the Bell 212 this is 11 Hz, which produce bodily discomfort in the pilots”.
Likewise, in an inspection performed among 802 pilots of the US Navy, 72.8% experienced one or more episodes of back discomfort while flying helicopters. According to the obtained data in the previously mentioned studies, vibrations produce a great variety of symptoms, since the data shows it should exist a minimal exposure to the flight duration before back pain is experienced. As a result, its determined that there exist two fundamental factors to take into account when developing the design of the new aircraft chairs and that these two factors are directly involved in the majority of the cases of back pain in helicopter pilots, which are the pilot's posture in flight and the exposure to the vibrations transmitted by the chair, the cyclic and the pedals.
Therefore, its made necessary to count with a device or system that allows to reduce the vibrations that are generated by the main rotor and the tail's rotor of a helicopter, without the need of modifying any of these two main components of the aircraft.
In this sense, in the state of the technique, there exist various types of vibration reduction systems to install in the helicopter's pilot chair. One of these is found in document EP 1392987, which refers to a shock absorber device to reduce the vibration in helicopters and that comprise a device that converts mechanical energy in electrical energy. This is mounted on a base plate joined to the aircraft's structure and its damping effects is based in the use of a suspended magnetic circuit in at least one spring, where all the device is installed in the main rotor of the helicopter.
One of the main disadvantages that are presented with the application of the invention described in the previous document is that the vibration reduction system is installed in the external part of the aircraft, specifically in the main rotor, which makes such device to have elevated installation and maintenance costs. Also, the risk is taken that it affects the behaviour of the blades given that it is made necessary the inclusion of an external device to the aircraft.
On the other part, the document EP 1659309 reports a damping apparatus to reduce the vibrations that are produced by the movement of the rotors in a helicopter, where such apparatus includes a piston that moves a hydraulic fluid inside a damping chamber, which resists the movement of the piston by means of the pressure of such fluid, which supplies damping forces that act to oppose to the movement of the piston reducing or cancelling the vibrations produced by the aircraft's rotors.
The main disadvantage presented by the described invention in the mentioned document is based in the complexity of the shock-absorbing system, given that it is made necessary the inclusion of a hydraulic fluid which use is not advisable for a helicopter, given that the chamber that contains such fluid can suffer blows that can crack it letting the fluid out and provoking problems in the helicopter's rotor.
Considering the previous information it is evident that it existed in the state of the technique the need to design and implement a device to reduce the vibrations produced by the rotors of a helicopter and that are transmitted directly to the pilot by means of his chair, where such device doesn't interfere with the flight mechanisms, that doesn't affect the pilots comfort, that is of easy installation and maintenance and that is also inexpensive, portable, and that can be installed in any type of helicopter and for any pilot, regardless of his weight and physical constitution.